A steam turbine is mainly composed of a high-pressure turbine to which main steam is guided, an intermediate-pressure turbine to which reheated steam is guided, and a low-pressure turbine to which steam exhausted from the intermediate turbine is guided.
For example, in the low-pressure turbine, an outer casing which is a pressure vessel is divided into two parts of an outer casing upper half and an outer casing lower half at a horizontal plane including the rotary shaft center line of a turbine rotor. A flange part of the outer casing upper half and a flange part of the outer casing lower half are fastened to each other by bolts or like.
A foot plate is provided to a side surface close to the flange part of the outer casing lower half. This foot plate is fixed to a foundation. The outer casing is supported on the foundation by the foot plate.
The low-pressure turbine is coupled to a condenser. Steam exhausted from the low-pressure turbine is condensed in the condenser so as to generate condensate.
Examples of an exhaust structure in the low-pressure turbine include a downward exhaust structure in which the condenser is disposed on the vertically lower side, an axial-flow exhaust structure in which the condenser is disposed on the axially downstream side, a side exhaust structure in which the condenser is disposed perpendicular and horizontal to the axial direction of the turbine rotor, and the like.
Among the above exhaust structures, the downward exhaust structure is more common as the exhaust structure used in the low-pressure turbine. The axial direction of the turbine rotor refers to a direction in which the shaft center line of the turbine rotor extends.
A connection method of connecting the low-pressure turbine and condenser is roughly classified into two. The first one is a method of flexibly connecting the low-pressure turbine and the condenser through an expandable member called “expansion”. The expansion is formed of, e.g., rubber, stainless, or the like.
The second one is a method of rigidly connecting the low-pressure turbine and the condenser by welding or bolt fastening. In this case, the low-pressure turbine and the condenser constitute one pressure vessel, so that they exert force according to an operation state to each other.
When the second method, i.e., the rigid connection method is adopted, the temperature in the low-pressure turbine and in the condenser rises at, e.g., the start-up of the turbine, to thermally expand the low-pressure turbine and condenser. At this time, reaction force to prevent the thermal expansion acts on a support part for the low-pressure turbine and condenser.
The inside of the outer casing of the low-pressure turbine is caused to be in a vacuum state by the condenser. Accordingly, the outer casing receives a load due to a difference between pressure applied to the outer surface thereof and pressure applied to the inner surface thereof. Typically, this load is called “vacuum load”.
In the downward exhaust structure, the vacuum load and reaction force due to thermal expansion and contraction vertically acts on the outer casing of the low-pressure turbine. The outer casing in the downward exhaust structure has, on the foundation, the foot plate having a large installation area and can thus receive the above load.
On the other hand, in a low-pressure turbine in the side exhaust structure provided with the condenser on one side of the outer casing, the load acts on the side at which the condenser of the outer casing is provided in directions perpendicular and horizontal to the axial direction of the turbine rotor.
FIG. 9 is a vertical cross-section view of a conventional low-pressure turbine 200 having the downward exhaust structure. FIG. 10 is a view illustrating an X-X cross section in FIG. 9.
As illustrated in FIG. 9, the low-pressure turbine 200 includes an outer casing 210, an inner casing 220 provided inside the outer casing 210, and a turbine rotor 230 penetrating the outer casing 210 and inner casing 220. In the inner casing 220, stationary blades 223 each supported between a diaphragm outer ring 221 and a diaphragm inner ring 222 and rotor blades 231 implanted to the turbine rotor 230 are alternately provided in the rotor axial direction.
A suction chamber 241 into which steam from a crossover pipe 240 is introduced is provided at the center of the low-pressure turbine 200. The introduced steam is distributed from the suction chamber 241 to left and right turbine stages.
On the downstream side of the final turbine stage, an annular diffuser 247 is formed by an outer peripheral side steam guide 245 and a cone 246 positioned on the inner peripheral side of the steam guide 245. The annular diffuser 247 exhausts steam radially outward.
As described above, the outer casing 210 is composed of an outer casing upper half 210a and an outer casing lower half 210b. As illustrated in FIG. 9, a pair of end plates 211 provided in the outer casing lower half 210b so as to extend perpendicular to the axial direction of the turbine rotor 230 each have a foot plate 212.
For example, the foot plate 212 extends perpendicular and horizontal to the axial direction of the turbine rotor 230. As illustrated in FIG. 9, the foot plate 212 is placed on a foundation 250 through, e.g., a sole plate 213. In this manner, the outer casing lower half 210b, i.e., outer casing 210 is supported on the foundation 250.
Although not illustrated, a pair of side plates provided in the outer casing lower half 210b so as to extend parallel to the axial direction of the turbine rotor 230 each also have a foot plate. This foot plate is also placed on the foundation 250.
Further, a bearing stand 260 is fixed onto the foundation 250 through, e.g., the sole plate 213. A bearing 261 supported on the bearing stand 260 is provided in a bearing casing 262. The turbine rotor 230 is rotatably supported by the bearing 261.
As illustrated in FIGS. 9 and 10, a center key 214 is provided on the foot plate 212 extending from the end plate 211. The center key 214 is disposed at the center of the width (width of the end plate 211 in directions perpendicular and horizontal to the axial direction of the turbine rotor 230) of the end plate 211. The center key 214 protrudes from the foot plate 212 to the bearing stand 260 side.
As illustrated in FIG. 10, a key fitting member 263 having a fitting groove 263a fitted to the center key 214 is fixed onto the end surface of the bearing stand 260 that is opposed to the center key 214. The center key 214 is integrally or detachably fixed to the foot plate 212.
Fitting the center key 214 to the fitting groove 263a of the key fitting member 263 allows alignment between the outer casing 210 and the turbine rotor 230 to be secured.
As described above, the fitting structure between the center key 214 and the key fitting member 263 is provided for securing the alignment. Therefore, as illustrated in FIG. 10, the center key 214 is formed of a member smaller in width (width in directions perpendicular and horizontal to the axial direction of the turbine rotor 230) and size. Further, such a fitting structure is positioned above the upper surface of the foundation 250.
As described above, in the low-pressure turbine having the side exhaust structure provided with the condenser on one side of the outer casing, the load acts on the outer casing in a direction perpendicular to the axial direction of the turbine rotor and in a direction horizontal to the side at which the condenser is provided.
Further, as described above, the fitting structure between the center key 214 and the key fitting member 263 in the conventional low-pressure turbine 200 having the downward exhaust structure is provided for securing the alignment.
Thus, when the above fitting structure is applied to the low-pressure turbine having the side exhaust structure, it is difficult for the fitting structure to bear the above load. When the fitting structure cannot bear the load, it may be broken to fail to maintain the outer casing at a predetermined proper position. This reduces reliability of turbine performance or turbine operation.